1. Field of the Invention
This invention relates to a wet-type segmented friction material that has opposite surfaces applied with a high pressure to generate torque while immersed in an oil. Such friction material may be made by joining by adhesion segment pieces made of a friction material substrate that was cut into a segment shape onto one or both of opposite surfaces of a core metal of a flat ring shape along a circumference thereof with an adhesion.
2. Description of the Related Art
As a recent wet-type friction material, a technique shown in FIG. 20 is under development in order to improve yield with resultant cost reduction and to lessen drag torque with resultant high fuel efficiency. FIG. 20 illustrates a plan view of an overall structure of a conventional segmented friction material. As shown in FIG. 20, the segmented friction material 221 has a core metal 222 of a flat ring shape and friction material substrates 223. The friction material substrates 223 are made by cutting a friction material blank into a segment piece shape. A set of such segment-shaped substrates 223 are arranged and joined with an adhesive onto a front surface of the core metal 222 side by side along a circumference of the core metal 222 with a gap to be an oil groove 224 formed therebetween. Another set of the segment-shaped substrates 223 are joined with the adhesive to a rear surface of the core metal 222, too. Such segmented friction material 221 is applicable to a friction material clutch device that has a single or plural friction plates and that is used in an automatic transmission of automobiles or motorcycles or the like. Such technique corresponds to a technique described in Japanese Laid Open Patent Publication No. 4-194422 or U.S. Pat. No. 5,094,331.
However, the conventional wet-type segmented friction material has drawbacks as follows. There are produced fuzz or scuffing at a cut surface of the substrates 223 when the friction material substrates 223 are cut off to become the segmented pieces. Then, the drag torque increases because the fuzz is generated to a large degree. Moreover, since the fuzz is generated to a large degree, strength of the segment piece or the substrate 223 decreases at the cut surface, thereby causing exfoliation of layers generated from the cut surface.
Moreover, an area around the oil groove 224 of the segmented friction material 221 is formed of the cut surface of the segment pieces 223. Therefore, there is a problem that a wall portion at the oil groove of the friction material 221 is vulnerable to exfoliation when it is applied with a load such as a shear torque by ATF (automatic transmission fluid) at an idle running time as compared with a friction material having a pressed oil groove that is a present mainstream.